A turbine engine includes essentially a compressor, a combustion chamber and a turbine, in which the compressor supplies the combustion chamber with pressurised air, and the turbine receives the hot gases coming from the combustion chamber so as to extract energy therefrom. The stage of the turbine located immediately downstream of the combustion chamber, commonly called the high-pressure stage, includes a disk equipped with blades on its periphery and surrounded in general by a stationary sealing ring held by a casing.
The radial clearance between the tips of the blades and the sealing ring must be as low as possible in order to minimise the passage of pressurised air outside of the zone swept by the blades and thus prevent the turbine performance from being hindered, but this radial clearance must be sufficient to avoid any friction of the tips of the blades on the sealing ring.
The radial clearance between the tips of the blades and the sealing ring is dependent on the thermal and mechanical expansions of the rotor and the thermal expansion of the stator of the turbine, in particular its casing and the sealing ring.
During the various phases of operation of a turbine engine, the radial clearance may vary considerably due to the movements of elements comprising the rotor and the stator of the turbine, and especially as these movements may be in opposite directions and be distributed non-homogeneously about the spindle of the turbine engine.
In particular, during operating cycles commonly called critical re-accelerations, in which the turbine engine goes, in a very short time, from a stabilised full rate of speed to a slowed rate of speed, and then returns quickly to the full rate of speed, the radial clearance passes through a minimum value. Indeed, the movements due to thermal expansion of the rotor disk are slow during the passage from the full rate of speed to the slowed rate of speed due to the large mass of the disk and the resulting long thermal response time; as the mass of the elements of the stator is lower, their thermal response is faster. Thus, in a sudden re-acceleration to the full rate of speed, the radial clearance is low between the rotor, which has not yet had time to be thermally stabilised at the slowed rate of speed, and the stator, which was able to reach the operation conditions on slowing. The centrifugal force resulting from the acceleration causes an additional expansion of the rotor, correspondingly reducing the radial clearance and capable of causing premature wear of parts if the tips of the blades have come into contact with the sealing ring.
It therefore appears that the faster the thermal response of the stator elements with respect to the response of the rotor, the smaller the radial clearance is between the tips of the blades and the sealing ring during a re-acceleration cycle, and the greater the risk is of premature wear.